Nitration of olefins



Dec- 9, 1952 T. F. DouMANl ETAL 2,621,205

NITRATION oF OLEFINS Filed Aug. l5, 1949 Patented ec. 9, i952 UNITED ST NITRATION F LEFINS ration of California Application August 15, 194.9, Serial No. 110,252

7 Claims.

This invention relates to the nitration of cycloolenes, and in particular concerns an improved process for preparing nitrogen oxide addition products of cycle-monoolenes, e. g. cyclohexene.

It is known that nitrogen oxide addition products of oleflne hydrocarbons, particularly those of the aliphatic series, may be obtained by reaction between nitrogen tetroxide and the olene under conditions of relatively low temperature. In general, however, such procedure is disadvantageous since it requires the use of highly purilied reactants, one of which is usually employed in large excess, as well as special reaction solvents, and requires a reaction time of several hours to secure only very moderate yields of the desired product. It is alsoknown to react olenes with nitrogen oxides in the vapor phase at temperatures' as high as 160 C. Such procedure, however, gives rise to oxidation as well as nitration reactions and results in the formation of a mixed product comprising a variety of unstable nitrogenand oxygen-containing cornpounds. Other means of nitrating oleflnes, as for example those in which nitric acid is employed as the nitrating agent, for the most part yield substitution products, i. e., products in which a nitrogen-containing group has been substituted for one or more of the hydrogen atoms of the olene, the double bond being retained.

It is accordingly an object of the present invention to provide an improved process for the preparation of nitrogen oxide addition products of cyclo-monoolenes.

Another object is to provide a process whereby nitrogen oxide addition products of cyclo-monoolenes are readily prepared in good yield and in a high state of purity.

A further object is to provide an improved procedure for eiecting reaction between nitrogen tetroxide and cyclo-olenes.

A still further object is to provide an improved method of preparing dinitro-cycloalkanes from the corresponding cycloalkenes, e. g., 1,2-dinitrocyclohexane from cyclohexene.

Other objects will be apparent from the following detailed description of the invention, and various advantages not specifically referred to herein will occur to those skilled in the art upon employment of the invention in practice.

We have found that the above and related objects may be realized in a process whereby nitrogen tetroxide is caused to react with cyclo-monooleflnes under conditions of moderately elevated temperature and pressure. In previously described processes for reacting nitrogen tetroxide with straightor branched-chain olei'lnes, the violent nature of the reaction and the instability of certain of the products have dictated the avoidance of elevated temperatures and pressures, especially the latter. We have found, however, that cyclo-monooleiines may be reacted with nitrogen tetroxide at temperatures as high as 300 C. and under pressures as high as atmospheres Without running undue risk of explosion, particularly if the reaction be carried out in the presence of a solvent and in a continuous manner as is more fully explained hereinafter. By operating under such conditions of moderately elevated temperature and pressure, the reactions whereby the desired addition products are formed take place relatively rapidly and are completed before undesirable side reactions, such as those involving oxidation, occur to any great extent. Operation under conditions which eiect completion of the desired reactions in a relatively short time is further advantageous in that it permits the use of readily available reaction solvents, e. g., certain saturated hydrocarbons Which ordinarily Would be considered not suitable for such use because of their tendency to react slowly with nitrogen tetroxide.

As is well known, nitrogen tetroxide exists in equilibrium with its monomer, nitrogen dioxide, in accordance with the equation:

Accordingly, it is to be understood that where the term nitrogen tetroxide is employed herein and in the appended claims, such term refers to the equilibrium mixture of nitrogen tetroxide and nitrogen dioxide that exists under the prevailing conditions of temperature and pressure. Similarly, it is to be understood that while the process of the invention is described herein as applied to the nitration of cycloheXene, it is also applicable to the nitration of other cyclo-monoolenes as cyclopentene, methyl-cyclohexene, dimethyl-cyclopentene, cycloheptene, etc.

Considering cyclohexene as typical of the cyclo-mono-oleines, the main reactions involved in the process of the invention are believed to Ibe as follows:

(l) Hz H2 /O\ /C\ H2O CH H2O CHN O N204 I u 2 HQC CH H2O HNO 2 C C H2 H2 Nitrogen Cyclohexeno 1,2-Dinitrocyc1ohexane Tetroxide (2) H2 Hz /C\ /C\ H2O CH H2O CH ONO N204 Il H2 /CH H2C\ CHNO 2 C O H2 H2 Nitrogen Cyclohexene Nitritonitrocyclohcxane Tctroxide It is also possible, particularly under conditions favorable to oxidation, for other reactions to occur with the formation of various nitrato-nitro, nitrito-nitroso, hydroxy-nitro, bis-nitroso and other compounds, but by operating in accordance with the present invention the formation of such products is greatly suppressed so .that thenitritonitro and dinitro compounds, particularly the latter, form the main products of the reaction.

The reaction between nitrogen tetroxide and cyclo-mono-olenes according to the invention is carried out inthe presence of the saturated hydrocarbon reactionl solvent corresponding to the cyclo-mono-olene reactant. Thus, when the cyclo-mono-oleine reactant is cyclohexene the reactant solvent is cyclohexane. Both reactants are initially dissolved in suchsolvent and the resulting solutions are separately preheated to approximately the desired reaction temperature before being contacted withl one another. It is preferred that the reaction be carried out continuously with the cyclo-mono-olene maintained in the liquid phase andv with concurrent ow of reactants.

WhenV operating according to such preferred procedure, the initial solution of the nitrogen tetroxide reactant may be obtained simply by dissolving relatively pure nitrogen tetroxide in the saturated hydrocarbon solvent. It is usually desirable, however, to take advantage of the fact that nitrogen tetroxide is more soluble in saturated hydrocarbons than are its normally incident impurities, e. g., -other oxides of nitrogen, air, ammonia, etc. Accordingly, the required nitrogentetroxide solution may be obtained directly from the gaseous mixture obtained in the preparation of nitrogen. tetroxide bythe oxidation of ammonia by extracting suchmixture with the saturated hydrocarbonsolvent. The preparation of nitrogen tetroxide by the air oxidation of ammonia at elevated temperatures in the presence of a catalyst such as platinum gauze is a well known procedure and gives rise to a` gaseous mixture comprising nitrogen tetro-xide in admixture with other oxides of nitrogen, water vapor, unreacted air and ammonia, and other impurities. Instead of processing such mixture by low temperature distillation, absorption or other means to separate pure nitrogen tetroxide, the gaseous mixture may simply be contacted with the saturated hydrocarbon solvent, as for example in a countercurrent extraction tower, whereby the nitrogen tetroxide is selectively dissolved to form a nitrogen tetroxide solution directly suitable for use according to the present invention.

The olene reactant may be derived from any convenient source, such as from petroleum rening operations or from other natural or synthetic sources. It may be employed in a relatively pure state or it may be in admixture with other olenes. In the latter case, mixed nitration products will be obtained. Also, if desired, the olene may be in admixture withthe corresponding saturated hydrocarbon. Thus, for example, a solution of cyclohexene in cyclohexane suitable for use in the present process may be obtained directly by fractionally distilling a naphthenic petroleum extract or distillate to separate a fraction boiling at about 75-85 C. The use of the saturated hydrocarbon solvent which corresponds to the olefine reactant is further advantageous in that should the solvent by any chance enter into the nitration reaction, the products thereby formed will be substantially the same as.,k those produced by nitration of the olene. Thus, in nitrating cyclohexene according to thev process of the invention, employing cyclohexane as the saturated hydrocarbon solvent, certain of the possible products of reaction between nitrogen tetroxide and the cyclohexene are identical with those of the reaction between nitrogen tetroxide and cyclohexene. Ordinarily, however, little of the saturated hydrocarbon solvent will react with the nitrogen tetroxide before the reaction between the nitrogen tetroxide and the cyclo-mono-oleine is substantially completed.

The manner in which the process of the inven tion is preferably carried out will be apparent-v from the accompanying drawing, the single figure of which represents a schematic flow sheet for the nitration of cyclohexene. Such mode of oper-- ation, however, is merely illustrativeand isnot to be construed as limiting the invention. Re-

ferring now to said drawing, a gas mixture-com prising nitrogen tetroxide, obtained for exampleby the air oxidation of ammonia,.is introducedv through line l into absorber 2 at a point near the: bottom, and cyclohexane is introduced into the: upp-er portion of the absorber through line 3.2

Within absorber 2, the gas mixturerises countercurrent to the descending stream of cyclohexarie` whereby the nitrogen tetroxide is selectively dissolved. NOn-absorbed gas leaves the absorber via line 5, and the solution of nitrogen tetroxide` in cyclohexane is withdrawn from the bottom of the absorber and is passed through line 5 to pump.

5 which supplies a pressure equal to that at which the reaction is to be carried out. From pump 6 the nitrogen tetroxide solution is passed through.

line l and heat exchanger 8, where it is heatexchanged against the hot reaction product, and

thence through line 9 to preheater. lil. Within the preheater, which may take the form4 of a; simple coil immersed in a molten salt bath, theV temperature of the nitrogen tetroxide solution is raised to approximately the value lat which the. reaction is to be carried out. From preheatera ll).

the heated nitrogen tetroxide solution passes through line ll into reactor l2.

The cyclohexene reactant enters the system through line i3 the reaction is carried out. From pump I6 the cyclohexene solution passes through lineV I1.- to heat exchanger i8, where it is heat-exchanged against the hot reaction through line to approximately the desired reaction temperature. Preheaters l0 and 2o may conveniently be heated by a common heat source, as shown. From preheater 2G, the cyclohexane solutionA is then passed through line 2l to reactor l 2.

Within reactor l2, which may also take the form of a coil immersed in a molten salt bath, the nitrogen tetroxide and cyclohexene solutions are combined and reaction occurs under the applied temperature and pressure conditions as previously explained. The resulting reaction product is withdrawn through line 22 and passes and is mixed with cyclohexane introduced via lines 3 and I4. The cyclohexene` product, and thence. l 9 to preheater20 which it is heated through pressure relief valve 23 to the heat exchangers 8 and I8 where part of its heat is given up to the incoming reactant streams. The product is then passed through line 2d to cooler 25 where it is cooled to substantially atmospheric temperature. From cooler 25 the product passes through line 2@ into separator 2'! where any noncondensed gases are separated and removed via line 28, and any water which may have been formed as a result of side reactions occurring is withdrawn through line 29. The crude product is then passed through line 30 into fractionator 3l where it is fractionally distilled to separate the cyclohexane solvent and the individual products of the nitration. As illustrated, such fractional distillation may yield an overhead stream of the cyclohexane solvent, a side-cut of nitritonitrocyclohexane, and a bottoms fraction comprising 1,2-dinitrocyclohexane.

Considering the various steps of the process described above in somewhat greater detail, the absorption operation effected in absorber 2 is preferably carried out at temperatures below about 40 C. down to about 10 C. or below, and under superatmospheric pressure, e. g. from about 50 to about 500 p. s. i. g. Sufficient hydrocarbon solvent is employed to dissolve the bulk of the nitrogen tetroxide from the gas mixture. The absorber may be of conventional design, such as a packed or bubble-cap tower, and preferably operates on the countercurrent principle. The nonabsorbed gas removed at the top of the absorber may be discarded or it may be treated for recovery of valuable constituents. Thus, if it contains an appreciable amount of nitric oxide and/or ammonia it may be oxidized to form nitrogen tetroxide for use in the process or it may be treated by conventional methods for the separation of the valuable constituents. The solution of nitrogen tetroxide in the hydrocarbon solvent which is withdrawn from the bottom of the absorber is usually of a concentration suitable for use directly in the nitration reaction, i. e., about l per cent by weight. If desired, however, such solution may be enriched with nitrogen tetroxide or diluted with the hydrocarbon solvent to obtain solutions of greater or less concentration. Ordinarily, it is preferable that the nitrogen tetroxide solution employed in the process contain from about 5 to about 25 per cent by weight of nitrogen tetroxide, although more highly concentrated solutions may be employed with the less reactive cyclo-olene reactants.

As an example of the operation carried out in the absorber, a gas mixture obtained by the air oxidation of ammonia and comprising about 6 per cent of nitrogen tetroxide, smaller amounts of nitric oxide, ammonia and water, and a large proportion of nitrogen is cooled to about l0 C. and compressed to about 100 p. s. i. g., and is passed countercurrent to a stream of cyclohexane in a packed absorption tower. The nonabsorbed gas withdrawn from the top ci the tower contains about 1 per cent or les-s of nitrogen tetroxide and the bulk of the nitric oxide, ammonia, water and nitrogen, while the cyclohexane solution withdrawn from the bottom of the tower contains about l0 per cent by weight of nitrogen tetroxide. Such solution may be employed directly in the process herein described. Similar absorption processes may be carried out with other saturated cycloaliphatic hydrocarbon solvents employing the same or somewhat diierent conditions to obtain suitable solutions of nitrogen tetroxide. Alternatively, such solutions may be prepared simply by dissolving relamono-olene reactant reacts but slowly with the nitrogen tetroxide, and in some instances the solvent may be omitted entirely.

The conditions which have been found most suitable for effecting the desired reaction include a reaction temperature between about 40 C. and about 300 C., preferably between about 80 C. and about 200 C., and a pressure between about 2 and about atmospheres suicient to maintain the cyclo-mono-oleiine and the solvent substantially entirely in the liquid phase. Usually the pressure will be between about 2 and about 20 atmospheres. It has also been found desirable to employ short contact times, e. g. from about 0.1 to 2 minutes, so as to minimize the formation of by-products either by side reactions between the nitrogen tetroxide and the cyclo-mono-olene or by reaction between the nitrogen tetroxide and the saturated hydrocarbon solvent. As previously explained, by-product formation is further minimized as a result of employing as the solvent the saturated cyclo-aliphatic hydrocarbon corresponding to the cyclo-mono-olefine reactant.

In order that the contact time at the desired temperature may be readily controlled, the reactants are heated approximately to the reaction temperature, i. e., between about 40 C. and about 300 C., prior to contacting one another in theA reactor. Such preheating step is preferably carried out after the reactants have been raised to the pressure at which the reaction is to be eifected. Any suitable form of preheater may be employed, and the two preheaters may advantageously consist of two separate coils mounted in a common vessel containing a suitable liquid or gas maintained at a suitable temperature. The reactor may likewise advantageously comprise a coil immersed in a liquid or gaseous medium heated to a suitable temperature. lt will be apparent that the design of the reactor will be determined by the velocity at which the reactants pass therethrough and the period of time over which it is desired that they remain in the reaction zone. The flow of the respective reactant streams to the reaction zone is in each case adjusted in accordance with the concentration of the reactant so that from about 0.75 to about 2.0 mols of nitrogen tetroxide are provided for each mole of cyclo-mono-olene. By employing mole ratios of this order, together with the previously mentioned short contact time, side reactions, as for example substitution and oxidation reactions and reactions between the nitrogen tetroxide and the solvent, are minimized.

The reaction product withdrawn from the reactor may contain a small amount of water and water-soluble by-products as well as small quantities of gas. Accordingly, it is desirable that the entire product be quickly cooled to a temperature below that at which further reaction takes place, and then treated t0 separate the organic products from any aqueous phase and/or non-condensed gases. Such treatmentmay comprise simple gravity settling whereby. the aqueous phase is separated as a subnatant layer, or it may involve other means of separation such as absorption, selective solvent extraction, or treatment with a dehydrating agent. L1 some cases the nature lof the product and itssubsequent purication is such that kthe separation step may be omitted entirely.

The crude nitration product is finally treated to separate the saturated hydrocarbon solvent, and such treatment-will usually also comprise separation of the diierent nitrated products formed in the reaction. The separation treatment may comprise a conventional fractional distillation, as is shown in the accompanying drawing, although other separation methods, such as crystallization, solvent extraction, azeotropic distillation, etc. may be employed il desired.

'The following example-will illustrate several ways in which the principle of the invention has been applied, but is not to be construed as limiting the same. All proportions are given in parts by weight.

Example Approximately 1770 parts cfa 17.41- per cent solution of nitrogen tetroxide in cyclohexane was passed at a rate of about 69 parts per minute and under a pressure of 20o p. s. i. g. through a preheater which consisted of an 1P- inch long stainless coil immersed in a liquid bath maintained at a temperature of about 155 C. From the preheater the solution `was passed directly into a reactor which consisted of a ifoot coil of lAl-inch stainless steel tubing immersed in a liquid bath maintained at a teinperature of about 160 C. cooled receiver was attached to the reactor coil so that the products issuing therefrom were immediately cooled to approximately room temperature. Simultaneously, 1375 parts of a 20.7 per cent solution of cyclohexene in cyclohexane was passed at a rate of about 58.5 parts per minute and under a pressure of about 200 p. s. i. through a similar preheater maintained at a temperature of 168 C. and into the reactor where it contacted the streaml of nitrogen tetroxide solution. The respective rates ci flow were such that the mole ratio of nitrogen tetroxide to cyclohexene was about 0.97/ and the time ci" passage through the reactor was about 0.26 minute. Upon coinpletion of the run, approximately 3102 parts of liquid product were recovered from the receiver. This product was steam-distilled without further treatment, whereby there was recovered 2346 parts of cyclohexane, an intermediate cut amounting to 21 parts, and 465 parts of a bottoms fraction comprising for the most part 1,2-dinitrocyclohexane and nitrite-nitrocyclohexane. Based on the amount of cyclohexene taken, the yield of nitrogen oxide addition products was about 74 per cent of theoretical.

The nitrogen oxide addition products prepared as described herein have varied uses in the arts. They are valuable chemical intermediates in the preparation of other chemical products, and may be employed as additives for various types of liquid hydrocarbon fuels, e. g., diesel fuel, jet propellants, etc.

Other modes of applying the principle of our invention may be employed instead of those' explained, change being made as regards the methods 'or materials employed provided the step or steps stated by any of the following claims,

or theequivalent 'of such step -orstepsnbeemployed.

We, therefore, particularly point VAout and distinctly claim as our invention:

l. The method of making nitrogen-oxide addition products cffcyclo-olenes which comprises forming-a solution of ya cyclo-mono-olene in -a solvent consisting essentially -of the cyclo-aliphatic hydrocarbon corresponding to said monoolene, forming a solution 'of nitrogen tetroxide in a solvent consisting essentially of the saturated cyclo-aliphatic hydrocarbon corresponding to said 'cyclo-mono-olene, separately heatingsaid solutions to 'a' temperature between aboutflO C. andabout 30SQ C. and under a pressure between about 2 and about 100 atmospheres suincient to maintain the cyclo-olonne and the solvent substantially entirely in the liquid phase,^c0mbining said solutions and maintaining the combined solutions at a temperature and a pressure within the aforementioned ranges for a period of time between about y0.1 and about 2 minutes, cooling the reaction product to a temperaturebelow that at which further reaction takes place, and recovering said addition products from said cooled reaction product.

2. The method of ina-king nitrogen oxide addition products of cyclohexene which comprises separately heating a solution of nitrogen tetroxide in cyclohexane and a solution of cyclohexene in cyclohexane to a temperature between about "V C. and about 200 C..and under ya pressure between about 2 and about 50 atmospheres suicient to maintin the cyclohexene and the cyclohexane substantially entirely in the' liquid phase, combining said solutions and maintainingsaid combined solutions at a reaction temperature betweenabout 80 Cyand about200" C. and under a pressure between about 2 and about 20 atmospheres suiicient to maintain the cyclohexene and the cyclohexane substantially entirely in the liquid phase for a period of time between about 0.1 and about 2 minutes, cooling the reaction product to a temperature below that at which further reaction takes place, and recoveringsaid addition products from said cooled reaction product.

3. The method of claim 2 wherein the nitrogen tetroxide and cyclchexene solutions contain between about 5 and about 25 per cent by weight of nitrogen tetroxide and cyclohexene, respectively.

4. The method of claim 2 wherein the nitrogen tetroxide and cyclohexene are employed in substantially equimolecular proportions.

5. The method of making nitrogen oxide addition products of cyclohexene which comprises continuously passing solutions of nitrogen tetroxide and cyclohexene in eyclohexane through separate preheating zones and into a common reaction Zone, the temperature within said preheating and reaction zones being maintained between about 80o C. and about 200 C. and the pressure within said preheating and reaction zones being maintained between about 2 and about 20 atmospheres and being sufficient to maintain the cyclohexene and the cyclohexane substantially entirely in the liquid phase, continuously passing said solution through `said reaction zone at such a rate that the residence `time within said reaction zone is between about 0.1 and about 2 minutes, continuously withdrawing a reaction product from said reaction zone and cooling said reaction product to a temperature below that at which further reaction takes place, and recovering said addition products from said reaction product.

tween about 5 and about 25 per cent by weight ofv nitrogen tetroxide and cyclohexene, respectively.

THOMAS F. DOUMANI. CLARENCE S. COE. EDWARD C. ATTANE, JR.

10 REFERENCES CITED The following references are of record in the le of this patent:

5 FOREIGN PATENTS Number Country Date 587,992 Great Britain May 12, 1947 603,344 Great Britain June 14, 1948 

1. THE METHOD OF MAKING NITROGEN OXIDE ADDITION PRODUCTS OF CYCLO-OLEFINES WHICH COMPRISES FORMING A SOLUTION OF A CYCLO-MONO-OLEFINE IN A SOLVENT CONSISTING ESSENTIALLY OF THE CYCLO-ALIPHATIC HYDROCARBON CORRESPONDING TO SAID MONOOLEFINE, FORMING A SOLUTION OF NITROGEN TETROXIDE IN A SOLVENT CONSISTING ESSENTIALLY OF THE SATURATED CYCLO-ALIPHATIC HYDROCARBON CORRESPONDING TO SAID CYCLO-MONO-OLEFINE, SEPARATELY HEATING SAID SOLUTIONS TO A TEMPERATURE BETWEEN ABOUT 40* C. AND ABOUT 300* C. AND UNDER A PRESSURE BETWEEN 